This invention concerns electrically pulsed infrared radiation sources, and discloses how, with simple means, one may specify and improve their performance far beyond what is possible in the prior art. This shall offer considerable advantages when utilizing such radiation sources in connection with gas sensors in particular, which by using infrared radiation sources according to the invention may be made better, simpler and less expensive compared to what has been possible earlier.
Infrared (IR) sensors for gas comprise both IR sources and IR detectors. IR detectors normally react only on changes in IR radiation. In connection with technical measurements this is traditionally established by pulsing the radiation by means of a so-called xe2x80x9cchopperxe2x80x9d, a rotating wheel with holes that chop the radiation from a constant (CW) source. This generates large temperature amplitudes between the hot source and the cold chopper blade, as seen from the IR detector. The radiation pulses may then be exactly calculated, because the temperatures will be known both for the IR source and the chopper blade. However, the pulses are locked onto one given frequency only, and the technique depends on expensive, inflexible and often delicate equipment with moving parts that are not adapted to modern electronic technology.
During the 80""s such chopped sources were to a large extent replaced by sources that could be electrically modulated, in which cases the source becomes heated by means of electrical pulses and is cooled by heat conduction into the immediate surroundings of the source. Only weak modulations of the source""s temperature may hereby be achieved, however, with amplitudes of the order of 1-10 K. As seen from the detector, the IR signals then become considerably weaker, with corresponding loss of sensitivity and resolution in technical measurements. Moreover, because the physical conditions concerning the heating and cooling of the source are in such cases not well defined, it also becomes difficult in advance to calculatexe2x80x94and to design the source forxe2x80x94the strength of the resulting IR signals.
IR detectors convert the IR signal into electrical signals with detector-specific responsivities R [V/W]. Next the electrical signals are subject to standard electronic amplification and signal treatment. The IR signal increases with the strength of the IR pulses. Therefore, large temperature amplitudes xcex94T for the source are essential. The measurements, however, are limited by noise, too, which is chiefly caused by the detector. In this respect, IR detectors are defined by their xe2x80x9cNoise-Equivalent-Powerxe2x80x9d, NEP, which increases with the square root of the electrical bandwidth xcex94f of the signal electronics; NEPxcx9c(xcex94f)1/2. In the most commonly used IR detectors, it is also the case that the noise increases in inverse proportion with the frequency at low frequencies, so-called 1/f noise, and only reaches a constant, low level at frequencies typically exceeding 500-1,000 Hz. When using chopped IR sources, this is solved by chopping at sufficiently high pulse rates, often of the order of 1,000 Hz or more.
With electrically modulated IR sources, however, that is not possible, because the modulation frequencies will be limited to ca 100 Hz, and even then with temperature amplitudes of the order of 1 K only. This results in small IR signals and large 1/f noise in existing, electrically modulated IR sensors.
The quality of an IR measurement is given by the signal-to-noise ratio S/N, which to good measure will be proportional to xcex94T and inversely proportional to (xcex94f)1/2; i.e., S/Nxcx9cxcex94T/(xcex94f)1/2. Given xcex94T and xcex94f, the properties of the sensor thus may be calculated rather exactly as a basis for its constructional making, design and manufacture. Within existing techniques one attempts to increase S/N by employing a small electric bandwidth in the measurement. This is made by including a narrow-band electronic filter into the detector electronics, that will pass electrical signals inside a very limited frequency band xcex94f only near the chosen pulse frequency f. For sensors with chopped IR sourcesxe2x80x94and for modulated IR sources in particular where the frequency is lower and the 1/f noise from the detector is higher and the bandwidth therefore has to be further narrowedxe2x80x94this in both cases implies that every single measurement shall take a long time, from seconds up to several minutes. Temporal resolution for time varying signals thus becomes inferior. Moreover, the measurements are carried out in continuous sequence, the sources are on all the time and draw a lot of current.
Added to the long time constants these are further deficiencies suffered by existing IR gas sensors. U.S. Pat. No. 5,220,173 opens for a possible solution to those problems, through its disclosure of an electrically pulsed thermal IR source which is cooled by thermal radiation between the pulses. Radiation-cooled IR sources may achieve temperature variations of the order of 100 K or more, with correspondingly strong IR pulses. The possibility then also exists to perform measurements by means of single pulses at chosen times, with the source turned off in between pulses. This may afford good temporal resolution and low current loads, with IR signals that approach those obtained with choppers. Said U.S. Patent, however, only provides the necessary conditions for the manufacture of a pulsed, radiation-cooled thermal IR source. The patent gives no answer as to how one may achieve temperature amplitudes of prescribed, preferred magnitudes, nor does it give any advice on which pulse lengths may be used. Such information is vital in order to produce IR sources whose performance and yield are determined from the requirements set by concrete applications, for example, when making real IR sensors which are optimized in relation to signal strength, temporal resolution, S/N ratio etc for a given technical measurement task.
The present invention takes as its starting point the said U.S. Pat. No. 5,220,173, and teaches how one may produce IR sources with powerful and entirely specified single IR pulses.
Suppose that the IR source is made from an electrically conductive foil shaped material, and that it radiates from a higher temperature Tm which is maintained for a certain element of time, subject to excitation by a pulse of electric current from a suitable electric drive circuit.
Normally the temperature is set depending on the spectral region which the source is to cover, often Tm may be found in the region 800-1,000 K. According to Planck""s law, the source then radiates with a power P=xcex5"sgr"Tm4 per unit square, where xcex5 less than 1 is the emissivity of the source surface and a "sgr"=5.67xc2x710xe2x88x9212 {dot over (W)} cmxe2x88x922 Kxe2x88x924 is the Stefan-Boltzmann constant. Compared with the radiation given off, the source receives little radiation in return from its surroundings, which are closer to room temperature. Likewise, suppose that the measurement task requires IR pulses of duration xcex8. With surface area A, and assuming that the source radiates equally to both sides, the IR source shall give off a net amount of radiative energy during the pulse that is, to good approximation, given by
Er=2Axcex5"sgr"xcex8Tm4xe2x80x83xe2x80x83(1)
When the current pulse is switched off, the source supposedly becomes rapidly cooled by a large, predetermined temperature amplitude xcex94T to a lower temperature T0, to produce a preferred magnitude of the IR signals. For the present IR sources according to the invention, such cooling occurs by means of thermal radiation, according to the said U.S. Pat. No. 5,220,173. The foil shaped material is assumed to have thickness d. Cooling the source across a temperature interval xcex94T from Tm to T0 then requires that an amount of thermal energy Es be removed from the source, where
Es=CAdxcfx81xcex94T,xe2x80x83xe2x80x83(2)
in which xcfx81 is the density and C is the specific thermal capacity of the source material.
A corresponding amount of energy must be supplied by the following current pulse in the form of electrical energy, in order to raise the source temperature from To back to Tm. For the IR source to satisfy the requirements of radiation cooling, according to the said U.S. Patent, the source must be made so as to make Er greater than Es. From this follows by simple calculation starting with (1) and (2), that the IR source must be made from a material with thickness d that obeys the relation
d less than 2xcex5"sgr"xcex8Tm4/Cxcfx81xcex94Txe2x80x83xe2x80x83(3)
Relation (3) discloses how pulsed, radiation-cooled IR sources must be made in order to make temperature amplitudes of xcex94T dimension feasible at pulse lengths xcex8. The necessary and sufficient physical requirements and conditions for producing radiation-cooled, pulsed IR sources thus are determined by those combinations of temperature amplitudes and pulse lengths which the sources are to produce. Relation (3) is new and entirely general, and is valid for any areal shape and size of the source, because all parts of the source surface are locally cooled by thermal radiation transversely to the surface and independently of heat conduction along the surface. When made according to relation (3), therefore, IR sources with large emitting surfaces may be pulsed as rapidly and as often as small sources; with previous, conduction-cooled sources this could not be done. Relation (3) is a characterizing feature of the present invention. According to the invention, the source may also be formed in such a way that the foil shaped material is either heated in its entirety, or that only certain parts of the source become heated to the actual temperatures; in any case it is the thickness of those parts of the source which are to be heated to the temperature Tm that occurs in relation (3).
The invention opens up for the manufacture of electrically pulsed IR sources which satisfy stated, preferred technical data and specifications, in a manner similar to IR detectors and other modern optical and electronic components and products. Next, this makes it possible to develop simple and inexpensive pulsed IR sensors that are adapted to particular applications, and whose performance may be calculated and specified even in the design phase. Such sensors by themselves represent a considerable novelty as compared with the prior art. The challenge will be to manufacture, to an industrial scale, such IR sources that can withstand those large thermal and mechanical strains which may arise, when they become subjected to frequent, repeated temperature excursions of 100 K or more. For most materials, this may bring about fracture due to buckling.
As another problem, the source may twist and bend as a result of heating, which produces an unstable source with shifting IR illumination of the IR detector. The invention has as its aim to provide solutions for all such tasks and problems, by creating a technological platform for industrial manufacture of improved IR sources, both as special, self-contained products and as essential, specified components in equipment that employs the source and its qualities in new and advanced methods and sensors for IR measurements of gas.